As the products of industry increased men began to exchange "the relatively superfluous for the relatively necessary," and trade in the form of barter became a permanent custom. This led to the use of money and a more extended system of exchange, and man entered the _commercial_ era. This gave him a wider intercourse with surrounding tribes and nations, and brought about a greater diversity of ideas. The excessive demand for exchangeable goods, the accumulation of wealth, and the enlarged capacity for enjoyment centred the activities of life in industry, and man entered the _industrial_ stage. At first he employed hand power for manufacturing goods, but soon he changed to power manufacture, brought about by discovery and invention. Water and steam were now applied to turn machinery, and the new conditions of production changed the whole industrial life. A revolution in industrial society caused an immediate shifting of social life. Classes of laborers in the great industrial army became prominent, and production was carried on in a gigantic way. We are still in this industrial world, and as electricity comes to the aid of steam we may be prepared for even greater changes in the future than we have witnessed in the past.[1] In thus presenting the course of civilization by the different periods of economic life, we must keep the mind free from conventional ideas. For, while the general course of economic progress is well indicated, there was a slow blending of each period into the succeeding one. There is no formal procedure in the progress of man. Yet we might infer from the way in which some writers present this matter that society moved forward in regular order, column after column. From the formal and forcible way in which they have presented the history of early society, one might imagine that a certain tribe, having become weary of tending cattle and goats, resolved one {40} fine morning to change from the pastoral life to agriculture, and that all of the tribes on earth immediately concluded to do the same, when, in truth, the change was slow and gradual, while the centuries passed away. It is well to consider that in the expanded industrial life of man the old was not replaced, but supplemented, by the new, and that after the pastoral stage was entered, man continued to hunt and fish, and that after formal agriculture was begun the tending of flocks and herds continued, and fishing was practised at intervals. But each succeeding occupation became for the time the predominant one, while others were relatively subordinate. Even to-day, while we have been rushing forward in recent years at a rapid rate, under the power of steam and electricity, agriculture and commerce have made marvellous improvement. Though we gain the new, nothing of the old is lost. The use of flocks and herds, as well as fish and game, increases each year, although not relatively.